The first changes the caption name for listings. The second — the header name for the list of listings which is printed by the \lstlistoflistings command. You would also probably want to modify the text in autorefs:

\def\lstlistingautorefname{Alg.}

Then when you're referring to a particular listing, e. g. For details see \autoref{sec2:bubblesort}. it is written as "For details see Alg. 2.1."